Coke deposition is generally experienced when hydrocarbon liquids and vapors contact the hot metal surfaces of petroleum processing equipment. The complex makeup of the hydrocarbons at elevated temperatures and contact with hot metal surfaces makes it unclear what changes occur in the hydrocarbons. It is thought that the hydrocarbons undergo various changes through either chemical reactions and/or the decomposition of various unstable components of the hydrocarbons. The undesired products of these changes in many instances include coke, polymerized products, deposited impurities and the like. Regardless of the undesired product that is produced, reduced economies of the process is the result. If these deposited impurities remain unchecked, heat transfer, throughput and overall productivity are detrimentally effected. Moreover, downtime is likely to be encountered due to the necessity of either replacing the affected parts or cleaning the fouled parts of the processing system.
While the formation and type of undesired products is dependent on the type of hydrocarbon being processed and the operating conditions of the processing, it may generally be stated that such undesired products can be produced at temperatures as low as 100.degree. F. However, the undesired products are much more prone to formation as the temperature of the processing system and the metal surfaces thereof in contact with the hydrocarbon increase. At these higher temperatures, coke formation is likely to be produced regard less of the type of hydrocarbon being charged. The type of coke formed, be it amorphous, filamentous or pyrolyric, may vary somewhat; however, the probability of coke formation is quite high.
Coke formation also erodes the metal of the system in two ways. The formation of catalytic coke causes the metal catalyst particle to become dislodged. This results in rapid metal loss and ultimately metal failure. The other erosive effect occurs when carbon particles enter the hydrocarbon stream and act as abrasives on the tube walls of the processing system.
As indicated in U.S. Pat. No. 4,962,264 which is herein incorporated by reference, coke formation and deposition are common problems in ethylene (olefin) plants which operate at temperatures of the metal surfaces are sometimes at 1600.degree. F. and above. The problem is prevalent in the cracking furnace coils as well as in the transfer line exchangers (TLEs) where pyrolytic type coke formation and deposition is commonly encountered. Ethylene plants originally produced simple olefins such as ethylene, propylene, buterie and butadiene from a feed of ethane, propane, butane and mixtures thereof. Later developments in this area of technology have led to the cracking of even heavier feedstocks to produce aromatics and pyrolysis gasoline as well as the light molecular weight olefins. Feed stocks now include kerosene light naphtha, heavy naphtha and gas oil. According to the thermal cracking processes utilized in olefin plants, the feedstocks are generally cracked in the presence of steam in tubular pyrolysis furnaces. The feedstock is preheated, diluted with steam and this mixture is then heated in the pyrolysis furnace to about 1500.degree. F. and above, most often in the range 1500.degree. F. to 1650.degree. F.
The effluent from the furnace is rapidly quenched by direct means or in exchangers which are designed to generate steam at pressures of 400 to 800 psig. This rapid quench reduces the loss of olefins by minimizing any secondary reactions. The cooled gas then passes to a prefractionator where it is cooled by circulating oil streams to remove the fuel oil fraction. In some designs, the gas leaving the oil is further cooled with oil before entering the prefractionator. In either case, the heat transferred to the circulating oil stream is used both to generate steam and to heat other process streams. The mixture of gas and steam leaving the prefractionator is further cooled in order to condense the steam and most of the gasoline product in order to provide reflux for the prefractionator. Either a direct water quench or heat exchangers are used for this post prefractionator cooling duty.
After cooling, cracked gas at, or close to atmospheric pressure, is compressed in a multistage compression system to much higher pressures. There are usually four or five stages of compression with interstage cooling and condensate separation between stages. Most plants have hydrocarbon condensate stripping facilities. Condensate from the interstage knockout drum is fed to a stripper where the C.sub.2 and lighter hydrocarbons are separated. The heavier hydrocarbons are fed to the depropanizer.
Accordingly, there is a need in the art to inhibit the formation and deposition of coke on surfaces in contact with high temperature hydrocarbons to improve the efficiencies of the processing system. Moreover, there is a particular need to retard coke formation and deposition during the high temperature pyrolysis and cracking of hydrocarbons.